Relay device, wireless communication system and multicast relay method

ABSTRACT

The present invention relates to a relay device realizing multicast communications efficient and effective in mobile communications. The relay device transmitting a multicast user data packet containing a content to a plurality of base stations comprises a storing unit storing, in a storing unit, information about a multicast group, information about the base station and information about the zone in a way that associates these items of information with each other, a determining unit determining, when receiving a LEAVE packet via the base station, whether or not it is required to transmit a query packet about a leave target multicast group by comparing the base station stored in the way being associated with the leave target multicast group and with the zone to which the via-base-station belongs with the base station relaying the received LEAVE packet, and a transmitting unit transmitting the user data packet containing the query packet to each of the base stations belonging to the zone on the basis of a result of the determination by the determining unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a relay device, a wirelesscommunication system and a multicast relay method that realize multicastcommunications efficient and effective in mobile communications.

2. Description of the Related Art

WiMAX (Worldwide Interoperability for Microwave Access) is defined as afixed wireless communication technology based on IEEE802.16.Standardization of the WiMAX technology for providing a fast wirelesscommunication service is in progress. The WiMAX Forum® is anindustry-led organization established as an organization for promotingthe WiMAX technology. Non-Patent document 1 given below is a documentissued by the WiMAX Forum® as a technical overview about the WiMAXtechnology.

Non-Patent document 1 describes a purport of supporting a multi BS (BaseStation)-MBS (Multicast Broadcast Service) in the WiMAX technology.Specifically, the wireless communication system using the WiMAXtechnology is requested to provide, through multicast communications, avariety of contents such as a news program, a sports program, a weatherforecast and traffic information.

IGMP (Internet Group Management Protocol) and MLD (Multicast ListenerDiscovery) are employed as multicast group management protocols in a waythat participates in and leaves a multicast group in the multicastcommunications using an IP (Internet Protocol) protocol. The IGMP is themulticast group management protocol that supports IPv4, and the MLD isthe multicast group management protocol that supports IPv6.

The following document is a conventional art related document thatdiscloses a system etc for supporting the multicast communications.Patent document 1 given below discloses a wireless multicast system etc.The wireless multicast system is that a router storing associativeinformation between information representing wireless access pointequipment that transmits the packet to the multicast group andinformation representing the multicast group, specifies the access pointequipment that should establish a connection to a wireless terminal bytransmitting the associative information to the wireless terminal.

Further, Patent document 2 given below discloses an IP packet multicastmethod. According to this method, in relay nodes having a hierarchicalstructure, a high-order node storing low-order node information inresponse to a participation request given from the low-order node anddeletes the information in response to a cancellation request.

The Patent document 1 is a “Japanese Patent Application Laid-OpenPublication No. 2004-15435”.

The Patent document 2 is a “Japanese Patent Application Laid-OpenPublication No. 2002-94562”.

The Non-Patent document 1 is a “WiMAX Forum, “A Technical Overview andPerformance Evaluation”, Mobile WiMAX-Part 1, Feb. 16, 2006”.

A wireless multicast system disclosed in Patent document 1 describedabove is not, however, suited to a system that supports mobilecommunications because of making the wireless terminal desiring toreceive the packet of the predetermined multicast group select theshould-be-connected wireless access point equipment.

Further, according to a multicast communication method disclosed inPatent document 2 given above, each of the base station (RNC) managesthe information on the participating multicast group regarding each ofthe connected mobile terminals, and hence such a problem arises that theinformation managed in each base station increases in quantity and needssynchronizing between the respective base stations, resulting inintricacy of the system architecture.

Further, Non-Patent document 1 does not disclose a specific method ofsupporting the multi BS-MBS in the WiMAX technology.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a relay device, awireless communication system and a multicast relay method that realizemulticast communications efficient and effective in mobilecommunications.

The present invention adopts the following configurations in order tosolve the problems. Namely, the present invention is related to a relaydevice transmitting a user data packet, containing a content to bedistributed to a mobile terminal, in which to set information about amulticast group associated with the content, to a plurality of basestations managed in the way of being grouped into zones, that comprisesa storing unit storing, in a storing unit, information about a multicastgroup in which the mobile terminal participates, information about thebase station performing wireless communications with the mobile terminaland information about the zone to which the base station belongs in away that associates these items of information with each other, anextracting unit extracting, when receiving a LEAVE packet of a requestfor leaving the multicast group from the mobile terminal via any one ofthe plurality of base stations, the information about the base station,which is stored in the way of being associated with the leave targetmulticast group and with the zone to which the via-base-station belongsfrom the storing unit, a determining unit determining whether or not itis required to transmit a query packet for checking existence of themobile terminal continuing the participation in the leave targetmulticast group by comparing the base station extracted by theextracting unit with the base station that relays the received LEAVEpacket, and a transmitting unit transmitting the user data packetcontaining the query packet to each of the base stations belonging tothe zone whose information is stored together with the information aboutthe leave target multicast group in the storing unit on the basis of aresult of the determination made by the determining unit.

In the relay device according to the present invention, the informationon the multicast group in which the mobile terminal participates ismanaged together with the information on the base station performing thewireless communications with the mobile terminal and with theinformation on the zone to which the base station belongs. In the caseof receiving the LEAVE message from the mobile terminal, the basestation stored in the storing unit in connection with the leave targetmulticast group and the zone to which the base station relaying thereceived LEAVE message belongs, is compared with the base stationrelaying the LEAVE message, and it is determined based on a result ofthis comparison whether or not the query packet needs transmitting tothe target zone related to the leave target multicast group.

With this scheme, according to the present invention, if determined tobe different as the result of this comparison, for example, it isdetermined that other mobile terminals participating in the leave targetmulticast group still exist in this zone, the query packet about theleave target multicast group can not be transmitted to each of the basestations belonging to this zone.

Therefore, according to the present invention, it is possible to reducethe futile transmission and reception of the message, for managing themulticast group, such as the query packet in that situation, and toperform the multicast communications efficient and effective in thesystem targeted at the mobile terminals on the premise that theseterminals are to move.

Further, the relay device according to the present invention may furthercomprise a updating unit updating, when receiving a JOIN packet of arequest for participating in the multicast group from the mobileterminal via any one of the plurality of base stations, the informationon the base station that is stored in the storing unit in the way ofbeing associated with the zone to which the via-base-station belongs andwith the participation target multicast group, with the information onthe base station that relays the JOIN packet.

Namely, according to the present invention, the participation status ofthe mobile terminal in the multicast group is managed in a way thatsequentially updates the information about the base station relaying theJOIN message for every zone to which the base station belongs each timethe JOIN message is relayed.

With this scheme, according to the present invention, if the basestation relaying the received LEAVE message is different from the basestation stored in the storing unit in the way of being associated withthe leave target multicast group and the zone, it can be determined thatother base stations wirelessly connected to other mobile terminalsparticipating in the multicast group exist in this zone. This caseeliminates a necessity of sending the query packet to this zone, andtherefore the futile messages can be reduced.

Further, the relay device according to the present invention may furthercomprise a deleting unit deleting from the storing unit, if a responsepacket is not received when the transmitting unit has transmitted theuser data packet containing the query packet, a record containing theleave target multicast group, the base station that relays the LEAVEpacket and the zone to which the base station belongs in the way oftheir being associated with each other.

Still further, the relay device according to the present invention mayfurther comprise a transferring unit checking, when the record isdeleted by the deleting unit, existence of other records stored in thestoring unit in the way of being associated with the multicast groupcontained in the deleted record, transferring the received LEAVE packetto a high-order device if no other records exist, and transferring noneof the received LEAVE packet if other records exist.

According to the present invention, the existence of the mobileterminals under other zones, which participate in the leave targetmulticast group related to the received LEAVE packet, can be checkedbased on the information stored in the storing unit, and hence the LEAVEpacket can be transferred to the high-order device only in the casewhere none of the mobile terminals participate in that multicast groupin all of the zones.

Therefore, according to the present invention, the futile messagesbetween the relay device and the high-order device can be eliminated,and hence the highly efficient multicast communications can be realized.

Yet further, the present invention is an invention related to a wirelesscommunication system comprising the relay device and the base stationdevice, and is also an invention related to a multicast relay method bywhich a computer etc is made to actualize any one of the functions ofthe relay device. Still further, the present invention may also be aprogram that realizes any one of the functions described above, and mayfurther be a readable-by-computer storage medium recorded with such aprogram.

According to the present invention, it is possible to provide the relaydevice, the wireless communication system and the multicast relay methodthat realize the multicast communications efficient and effective in themobile communications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a system architecture of a multi BS-MBSsystem in the present embodiment;

FIG. 2 is a diagram illustrating a functional configuration of an ASNgateway;

FIG. 3 is a diagram showing an example of a MBS zone information tablewithin a MBS zone management database;

FIG. 4 is a diagram showing a multicast group information table withinthe multicast group management database;

FIG. 5 is a diagram showing an example of a tunnel information tablewithin a tunnel information database;

FIG. 6 is a diagram showing a JOIN message sent to an ASN gateway from abase station;

FIG. 7 is a diagram showing the JOIN message sent to an edge router fromthe ASN gateway;

FIG. 8 is a diagram showing a membership query (Group Specific) messagesent to the base station from the ASN gateway;

FIG. 9 is a diagram showing a membership report message sent to the ASNgateway from the base station as a response to the GS-query message;

FIG. 10 is a diagram showing a LEAVE message sent to the ASN gatewayfrom the base station;

FIG. 11 is a diagram showing the LEAVE message sent to the edge routerfrom the ASN-gateway;

FIG. 12 is a diagram showing multicast data transmitted from a CSN anddistributed to mobile terminals via the edge router, the ASN gateway andthe base station;

FIG. 13 is a diagram showing a process when receiving the JOIN message;

FIG. 14 is a diagram showing a process when sending the GS-querymessage;

FIG. 15 is a diagram showing a first process pattern when receiving aLEAVE message; and

FIG. 16 is a diagram showing a second process pattern when receiving theLEAVE message.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment

A multi BS-MBS system (which will hereinafter simply be referred to as aMBS system) in an embodiment of the present invention will hereinafterbe described with reference to the drawings. It should be noted that aconfiguration in the following embodiment is an exemplification, and thepresent invention is not limited to the configuration in the embodiment.Further, the following discussion will deal with only a multicastcommunication service by the MBS system in the present embodiment,however, the MBS system may include other functions.

System Architecture and Device Configuration

A system architecture of the MBS system in the present embodiment willbe explained with reference to FIG. 1. FIG. 1 is a view illustrating thesystem architecture of the MBS system in the present embodiment.

The MBS system in the present embodiment is built up by connecting a CSN(Connectivity Service Network) 10 and an ASN (Access Service Network) 15to each other via an edge router 20. Note that the MBS system in thepresent embodiment may be configured to include a plurality of CSNs 10and a plurality of ASNs 15 respectively, and the individual networks maybe connected with no intermediary of the edge router 20.

CSN

The CSN 10 is configured by connecting a router, an AAA (AuthenticationAuthorization Accounting) proxy server, a user database, an interworkinggateway, etc, respectively (unillustrated). The CSN 10 provides an IP(Internet Protocol) connection service etc for a WiMAX subscriber (e.g.,a mobile terminal 60) provided with a WiMAX service. Further, the CSN 10provides a variety of contents such as a news program, a sport program,a weather forecast and traffic information as the WiMAX service throughthe multicast communications. The respective contents to be provided areassigned multicast addresses different from each other, and the CSN 10manages a relative table between the respective contents and themulticast addresses. This relative table may be retained adjustablybeforehand in the memory etc of the CSN 10. The description of thecommunication service provided by the CSN 10 is about a service based onthe multicast communications, however, the MBS system can provide othertypes of communication services.

The CSN 10 is connected to the ASN 15 via the edge router 20. Thisconnection involves utilizing, e.g., the IP. The CSN 10 transmits, tothe edge router 20, a should-be-provided content as a multicast packethaving a setting of a multicast address assigned with respect to thecontent.

Edge Router

The edge router 20 is connected to a plurality of ASN gateways 30 undermanagement and manages information on the multicast communications fordistributing the content transmitted from the CSN 10. The edge router 20treats each of the ASN gateways 30 under the management as a hostparticipating in a multicast group. The edge router 20 independentlyprocesses a multicast group management message, thereby controlling aprocedure for each ASN gateway 30 to participate in or leave thepredetermined multicast group.

ASN

The ASN 15 is configured by connecting the ASN gateway 30 and the basestations 50. The mobile terminal 60 performs wireless communication withthe base station 50 covering a communication-enabled area where themobile terminal 60 exists, thereby connecting the MBS system accordingto the present embodiment and thus getting provided with the WiMAXservice. Each of the devices constructing the ASN 15 will hereinafter bedescribed.

ASN Gateway

The ASN gateway 30 is a gateway device including a CPU (CentralProcessing Unit), a memory, an input/output interface, etc. The ASNgateway 30 is connected to the plurality of base stations 50 undermanagement. An example in FIG. 1 is that the ASN gateway 30 is connectedto each of base stations BS1, BS2, BS3, BS4 and BS5. The ASN gateway 30groups the base stations 50 under the management into predetermined MBSzones, thus controlling the multicast communications according to theMBS zone. The example in FIG. 1 shows that the base stations BS1, BS2and BS3 are managed as a MBS zone #1, and the base stations BS4 and BS5are managed as a MBS zone #2.

A device configuration of the ASN gateway 30 will hereinafter beexplained with reference to FIG. 2. FIG. 2 is a block diagramillustrating the functional configuration of the ASN gateway 30. The ASNgateway 30 has a high-order communication unit 201, a multicast controlunit 202, a MBS zone management unit 203, a multicast group managementunit 204, a tunneling control unit 208, a MBS zone management database205, a multicast group management database 206, a tunnel informationdatabase 209, etc.

The high-order communication unit 201 controls the multicastcommunications with the edge router 20. To be specific, the high-ordercommunication unit 201 receives the multicast packet containing thecontent information transmitted from the edge router 20, and transfersthe received multicast packet to the multicast control unit 202.Further, the high-order communication unit 201 transmits a packet formulticasting to the edge router 20. Note that the high-ordercommunication unit 201 may process data other than the multicast packet.

The multicast control unit 202 works in cooperation with the high-ordercommunication unit 201, the MBS zone management unit 203, the multicastgroup management unit 204 and the tunneling control unit 208, therebycontrolling the multicast communications for distributing the contentinformation transmitted from the CSN 10 to the mobile terminals 60. Eachmobile terminal 60 needs to participate in the predetermined multicastgroup in order to receive distribution of a desired content. Themulticast control unit 202 controls the procedure for each mobileterminal 60 to participate in or leave the predetermined multicastgroup. For instance, the multicast control unit 202 controls IGMP(Internet Group Management Protocol) or MLD (Multicast ListenerDiscovery) utilized as the procedure described above. The multicastcontrol unit 202 processes a JOIN message (membership report message), aLEAVE message, a membership query (Group Specific) message (which willhereinafter be referred to as a GS-query message), a membership query(General) message (which will hereinafter be termed a G-query message),etc as multicast group management messages used by the protocol givenabove.

Moreover, the multicast control unit 202 processes the multicast datacontaining the content information transmitted from the CSN 10. Themulticast control unit 202 transfers, in the multicast group managementmessage and the multicast data, the multicast packet that should betransmitted to the edge router 20 to the high-order communication unit201 and transfers the multicast packet that should be transmitted to themobile terminal 60 to the tunneling control unit 208 together with theinformation about the MBS zone serving as a transmission destination.In-depth descriptions of the multicast group management message and themulticast data, which are to be processed in the multicast control unit202, will be made later on.

The MBS zone management unit 203 refers to the MBS zone managementdatabase 205 and thus manages the information on the zones to which therespective base stations 50 belong. The MBS zone management unit 203refers to a MBS zone information table within the MBS zone managementdatabase 205. FIG. 3 is a diagram showing an example of the MBS zoneinformation table within the MBS zone management database 205. The MBSzone information table retains pieces of information on the MBS zones towhich the respective base stations belong. The MBS zone informationtable may be previously set up and may also be updated dynamicallythrough the communications.

The multicast group management unit 204 manages, based on the multicastgroup management message processed by the multicast control unit 202,the respective pieces of information about the multicast groups in whichthe mobile terminals 60 within the MBS zones participate with respect toevery MBS zone. This management involves making use of the multicastgroup management database 206.

FIG. 4 is a diagram showing a multicast-group information table withinthe multicast group management database 206. The multicast groupmanagement unit 204 retains, for every multicast group and every MBSzone, a multicast address, MBS zone information and last responderinformation in the multicast group information table. What is retainedas the last responder information is information about the base stationthat relayed last a request for the participation (JOIN message) in thesame multicast group within the same MBS zone.

The tunneling control unit 208 controls how a GRE (Generic RoutingEncapsulation) tunnel is generated and cancelled, and processes themulticast communications with the base station 50 via the GRE tunnel.Specifically, the tunneling control unit 208 transmits the multicastpacket sent from the multicast control unit 202 to the predeterminedbase station 50 via the GRE tunnel. At this time, the tunneling controlunit 208 similarly, if the multicast packet is either the multicastgroup management message or the multicast data, transmits the multicastpacket to the predetermined base station 50 via the GRE tunnel. Further,the tunneling control unit 208 receives a frame sent via the GRE tunnelfrom the base station 50 and transfers the received frame data to themulticast control unit 202. In the MBS system, when the multicast groupmanagement message and the multicast data are transferred and receivedbetween the CSN 10 and the mobile terminal 60, the communicationsbetween the base station 50 and the ASN gateway 30 involve using the GREtunnel. Note that details of the GRE tunneling protocol are as definedby RFC2784 and RFC2890, and its description is herein omitted.

The tunneling control unit 208 at the initial operation time generates,based on the information in the tunnel information database 209, adownlink GRE tunnel utilized for the multicast communications flowing toeach base station 50. The downlink GRE tunnels utilized for thecommunications flowing to the base stations 50 from the ASN gateway 30are generated by, e.g., a numerical value obtained from multiplying thenumber of the base stations 50 connected to the ASN gateway 30 by thenumber of the contents to be provided.

FIG. 5 is a diagram showing an example of a tunnel information tablewithin the tunnel information database 209. The tunnel information tablestores items of information about the should-be-generated GRE tunnels.The items of information stored in this tunnel information table are aGRE key defined as an identifier, a multicast address utilizing the GREtunnel and should-be-generated destination MBS zone information (ZONE#1,ZONE#2, ZONE#3 in FIG. 5) with respect to every GRE tunnel.

The tunneling control unit 208, when receiving the should-be-transmittedmulticast packet and the information on the transmitting destination MBSzone from the multicast control unit 202, requests the MBS zonemanagement unit 203 to acquire the information about the base stations50 belonging to the transmitting destination MBS zone. The tunnelingcontrol unit 208 copies the multicast packet by a number correspondingto the acquired number of the base stations 50. The tunneling controlunit 208 attaches a predetermined header such as a GRE header to thecopy of the multicast packet, and thereafter transmits thethus-generated frame via the GRE tunnel established between each of thebase stations 50 and the ASN gateway 30.

At this time, the tunneling control unit 208 sets timing informationassociated with the transmitting destination MBS zone in this GREheader. The timing information is used for the individual base stations50 belonging to the same MBS zone to simultaneously wirelessly transmitthe same message sent from the ASN gateway 30. Further, the timinginformation is generated from maximum delay time in each MBS zone, whichis calculated based on processing delay time, communication delay time,etc collected from each of the base stations 50 belonging to theindividual MBS zones by, e.g., an unillustrated function unit. It shouldbe noted that the present invention does not limit a method ofgenerating the timing information, and this timing information may beenough if used for the same multicast data to be simultaneouslytransmitted through the wireless communications from the base stationsbelonging to the same MBS zone.

Each of the messages processed by the multicast control unit 202 willhereinafter be described with reference to FIGS. 6 through 12.

FIG. 6 is a diagram illustrating the JOIN message sent to the ASNgateway 30 from the base station 50. The JOIN message is a message sent,on the occasion of participating in the multicast group, from the mobileterminal 60 in a way that assigns a multicast address of awant-to-participate-in multicast group, and a message type of the JOINmessage is the same as of a membership report message. FIG. 6illustrates a frame structure when the base station 50 receiving theJOIN message from the mobile terminal 60 transmits this JOIN message viathe GRE tunnel. The frame is formed by attaching predetermined headers302 (the GRE header, an IP header and a L2 (Layer-2) header) to amembership report packet 301 sent from the mobile terminal 60. Amulticast address of the want-to-participate-in multicast group isentered in a group address field of the membership report packet 301.

The multicast control unit 202, upon receiving the JOIN message, refersto a SA (Sender Address) field of the IP header and thus specifies thesender base station. The multicast control unit 202 acquires the MBSzone, to which the specified base station belongs, from the MBS zonemanagement unit 203. The multicast control unit 202 extracts themulticast address from the group address field in an IGMP part of thereceived membership report packet 301. The multicast control unit 202requests the multicast group management unit 204 to check whether or notthe extracted multicast address has already been stored in the multicastgroup information table.

If the multicast address has already been stored in the multicast groupinformation table with respect to the MBS zone, the multicast controlunit 202 transfers, to the multicast group management unit 204, therespective pieces of information about the sender base station 50 andthe MBS zone to which the sender base station 50 belongs. The multicastgroup management unit 204 updates, based on these pieces of information,the last responder information into the information about the senderbase station 50 in a record of the target multicast address and thetarget MBS zone within the multicast group information table.

Note that if only the records of the different MBS zones exist even whenthe target multicast address is stored, the multicast group managementunit 204 adds a new record to the multicast group information table andthus registers these pieces of information.

Whereas if the multicast address is not yet set in the multicast groupinformation table, the multicast control unit 202 extracts themembership report packet 301 from the frame, then sets an IP address ofthe self-device (the ASN gateway 30) in the SA field of the IP headerpart of the extracted packet, and transfers this packet to thehigh-order communication unit 201 so as to forward the same packet tothe edge router 20. FIG. 7 is a diagram showing the JOIN message sent tothe edge router 20 from the ASN gateway 30. As shown in FIG. 7, themembership report packet, in which the ASN gateway 30 is set as Sender,is attached with the L2 header and is then transmitted to the edgerouter 20.

Next, FIG. 8 is a diagram illustrating a membership query (GroupSpecific) message transmitted to the base station 50 from the ASNgateway 30. The GS-query message is a message sent for checkingexistence of the mobile terminal 60 (receiving the multicast message ofwhich a specified multicast address is designated) participating in aspecified multicast group. The ASN gateway 30 sends the GS-query messagerelated to each multicast group set in the multicast group informationtable to each of the MBS zones set in the same table.

At this time, the multicast control unit 202 generates a GS-query packet701 as illustrated in FIG. 8. In the GS-query packet 701, an existencechecking target multicast address is set in a group address field of theIGMP part and in a DA (Destination Address) field of the IP header part.The multicast control unit 202 transfers the generated GS-query packet701 to the tunneling control unit 208. The tunneling control unit 208copies the GS-query packet 701, corresponding to the number of the basestations belonging to the transmitting destination MBS zone, thenattaches predetermined headers 702 to each of the copies of the GS-querypacket 701, and transmits each of the thus-generated frames to theindividual base stations 50 via the GRE tunnels. At this time, thetunneling control unit 208 sets the timing information associated withthe transmitting destination MBS zone in the GRE header part of theheaders 702.

FIG. 9 is a diagram showing the membership report message sent to theASN gateway 30 from the base station 50 as a response to the GS-querymessage. The membership report message is a message sent from the mobileterminal 60 if the mobile terminal 60 receiving the GS-query messagecontinues to participate in the multicast group set therein. FIG. 9illustrates a frame structure on such an occasion that the base station50 receiving the membership report message from the mobile terminal 60sends this message via the GRE tunnel. This frame is formed by attachingpredetermined headers 802 (a GRE header, an IP header and a L2 header)to a membership report packet 801 transmitted from the mobile terminal60. A multicast address continuing its participation is set in a DAfield and in a group address field of the membership report packet 801.

The multicast control unit 202, upon receiving the membership reportmessage, checks the existence of the mobile terminal 60 continuing theparticipation about the multicast address set in the group address fieldof the IGMP part of the membership report packet 801. Thereafter, themulticast control unit 202 transfers the membership report packet 801 tothe tunneling control unit 208. The tunneling control unit 208 attachespredetermined headers to the membership report packet 801, and transmitsthe thus-generated frame via the GRE tunnels to the individual basestations 50 belonging to the MBS zone, which are registered in themulticast group. Owing to this scheme, the multicast control unit 202recognizes that the mobile terminals 60 existing in the same MBS zoneand participating in the same multicast group have no necessity ofsending the membership report message any more.

While on the other hand, the multicast control unit 202, if receivingnone of the membership report message for a predetermined period oftime, recognizes that there exist none of the mobile terminals 60participating the multicast group in that MBS zone, and notifies themulticast group management unit 204 of this purport. The multicast groupmanagement unit 204 deletes the record related to the multicast groupabout the MBS zone concerned. Note that the multicast control unit 202may retransmit, in the case of recognizing that any mobile terminal 60participating in the multicast group does not exist, the GS-querymessage a plural number of times.

Next, FIG. 10 is a diagram illustrating a LEAVE message sent to the ASNgateway 30 from the base station 50. The LEAVE message is a message sentfrom the mobile terminal 60 on the occasion of leaving the participatingmulticast group. FIG. 10 illustrates a frame structure on such anoccasion that the base station 50 receiving the LEAVE message from themobile terminal 60 sends the LEAVE message via the GRE tunnel. Thisframe is formed by attaching predetermined headers 902 (a GRE header, anIP header and a L2 header) to a LEAVE packet 901 transmitted from themobile terminal 60. A multicast address of a want-to-leave is entered ina group address field of the LEAVE packet 901.

The multicast control unit 202, when receiving the LEAVE message,specifies the sender base station by referring to the SA field of the IPheader. The multicast control unit 202 acquires the MBS zone, to whichthe specified base station belongs, from the MBS zone management unit203. The multicast control unit 202 extracts the multicast address fromthe group address field of the IGMP part of the received LEAVE packet901. The multicast control unit 202 requests the multicast groupmanagement unit 204 to check based on the extracted multicast addresswhether the last responder in the MBS zone is identified with the senderbase station or not.

The multicast control unit 202, if the sender base station is notidentical with the last responder, directly terminates the process onthe assumption that there exist other mobile terminals 60 participatingin the multicast group. Whereas if the sender base station is the lastresponder, the multicast control unit 202 sends the GS-query message tothe MBS zone where this base station exists in order to check whetherother mobile terminals 60 participating in the multicast group exist inthis MBS zone or not. The process of sending the GS-query message is thesame as described above. Through this process, it is confirmed thatthere exists no other mobile terminal 60 participating in the LEAVEmessage target multicast group, and a record related to this multicastgroup is deleted from the multicast group information table.

The multicast control unit 202, when recognizing from the LEAVE messagethat the record related to the multicast group concerned is deleted fromthe multicast group information table, in other words, that there existsnone of the mobile terminal 60 participating in this multicast group inthe MBS zone concerned, extracts the LEAVE packet 901 from the frame,then enters an IP address of the self-device in the SA field of the IPheader part of the extracted packet, and transfers the packet to thehigh-order communication unit 201 so as to forward this packet to theedge router 20. FIG. 11 is a diagram illustrating the LEAVE message sentto the edge router 20 from the ASN gateway 30. As illustrated in FIG.11, the LEAVE packet, in which the ASN gateway 30 is set as Sender, isattached with the L2 header and is then sent to the edge router 20.

FIG. 12 is a diagram showing multicast data transmitted from the CSN 10and distributed to the mobile terminal 60 via the edge router 20, theASN gateway 30 and the base station 50. Multicast data 1201 transmittedfrom the CSN 10, of which a multicast address associated with contentinformation contained therein is set, is sent to the ASN gateway 30 viathe edge router 20. The high-order communication unit 201 of the ASNgateway 30, when receiving the multicast data, transfers this data tothe multicast control unit 202.

The multicast control unit 202 extracts the multicast address set in theDA field of the multicast data, and requests the multicast groupmanagement unit 204 for information about the MBS zone where the mobileterminal 60 participating in the multicast address exists. The multicastcontrol unit 202 sends the information about the MBS zone, of which themulticast group management unit 204 notifies, to the tunneling controlunit 208 together with the multicast data 1201.

The tunneling control unit 208 copies the multicast data 1201 by a countcorresponding to the number of the MBS zones and corresponding to thenumber of the base stations belonging to each of the MBS zones, attachespredetermined headers 1202 to each of the copies of the multicast data1201, and transmits the thus-generated frames to the respective basestations 50 via the GRE tunnels. At this time, the tunneling controlunit 208 sets the timing information corresponding to the transmittingdestination MBS zone in the GRE header part of the headers 1202. Therespective base stations 50 receiving this frame within the same MBSzone wirelessly transmit the multicast data 1201 of the frame at thesame timing specified by the timing information. The MBS systemaccording to the present embodiment attaches the same headers to themulticast data when transmitted to the base station and to the previousmulticast group management message also when transmitted to the basestation.

Base Station

The base station 50 includes a CPU (Central Processing Unit), a memory,an input/output interface, etc, and the mobile terminals 60 within acommunication area covered by the base station 50 are connected to theMBS system through the wireless communications. Further, the basestation 50 is connected to the predetermined ASN gateway 30 by a cable,forwards the signals (the multicast packet) transmitted from the mobileterminal 60 to the ASN gateway 30 via the GRE tunnel, and wirelesslytransmits the multicast packet sent via the GRE tunnel from the ASNgateway 30 to the mobile terminal 60. Note that a process ofestablishing and cancelling the wireless link between the base station50 and the mobile terminal 60 and a process of assigning the IP addressare the same as by the well-known technologies, and hence theirexplanations are omitted.

The base station 50 retains information (e.g., an IP address etc) aboutthe ASN gateway 30 that manages the self-device. The base station 50 atthe initial operating time generates an uplink GRE tunnel employed forthe multicast communications with the ASN gateway 30 by use of the GREtunneling protocol defined by RFC2784 and RFC2890.

The base station 50, when receiving the membership report packet 301(see FIG. 6) (a response message to the JOIN message and to the GS-querymessage) and the LEAVE packet 901 (see FIG. 10) transmitted from themobile terminal 60, generates the frame by attaching the predeterminedheaders 302 (the L2 header, the IP header and the GRE header) to thesepackets, and transmits this frame to the ASN gateway 30. At this time,the base station 50 sets an IP address of the self-device in the SAfield of the IP header and sets an IP address of the ASN gateway 30 inthe DA field of the IP header.

The base station 50, in the case of receiving the frame transmitted viathe GRE tunnel from the ASN gateway 30, extracts the multicast packet(the GS-query packet 701 (see FIG. 8), the multicast data 1201 (see FIG.12)) from the frame. Subsequently, the base station 50 sends theextracted multicast packet through a predetermined wireless channel atthe time specified by the timing information set in the GRE header ofthe same frame. The timing information set in the multicast packettransmitted via the GRE tunnel from the ASN gateway 30 and in the GREheader, is the same on an every-MBS-zone basis. With this setting of thetiming information, the base stations 50 belonging to the same MBS zonetransmit the same multicast packet almost simultaneously.

Mobile Terminal

The mobile terminal 60 is a terminal device including a CPU (CentralProcessing Unit), a memory, an input/output interface, etc and isexemplified such as a mobile phone, a PDA (Personal Digital Assistant)and a personal computer. The mobile terminal 60 has a multicastcommunication function and a wireless communication function. The mobileterminal 60 performs the wireless communications with the base station50 that covers a communication area embracing a location of the mobileterminal, corresponding to where the mobile terminal 60 exists. Themobile terminal 60 receives a variety of contents distributed bymulticasting from the CSN 10 via the base station 50 with which thewireless link is established. Each mobile terminal 60 participates inthe multicast group for a desired content, thereby receiving eachcontent.

The mobile terminal 60 may download and thus retain acontent-to-multicast-address relative table held previously by the CSN10 in order to participate in a desired multicast group. Further, themobile terminal 60 may also previously acquire an IP address forconnecting to the MBS system. The present invention does not limit themethod of knowing the multicast address of the content, the method ofacquiring the IP address, etc, and hence their explanations are hereinomitted.

Operational Example

An operational example of the MBS system in the present embodiment willhereinafter be described with reference to FIGS. 13 and 14.

The description using FIG. 13 will start with the operation in the caseof transmitting the JOIN message from the mobile terminal 60. FIG. 13 isa sequence diagram showing a process when receiving the JOIN message,wherein the system architecture illustrated in FIG. 1 is exemplified.Exemplified is a case in which as the base stations 50, the BS1, BS2 andBS3 are defined to belong to the MBS zone #1, the BS4 and BS5 aredefined to belong to the MBS zone #2, the MS1 as the mobile terminal 60exists within the communication area of the BS2, and the MS3 and MS4 asthe mobile terminals 60 exist within the communication area of the BS5.

The MS1 desires to participate in a multicast group (224.22.3.45), andwirelessly transmits the JOIN message in which the multicast address(224.22.3.45) is set (S1301).

The BS2, upon receiving the JOIN message signal, extracts a membershipquery report packet from this signal. The BS2 transmits a framegenerated by attaching predetermined headers to the membership queryreport packet to the ASN gateway 30 via the GRE tunnel (S1302). At thistime, the BS2 sets an IP address of the base station itself in the SAfield within the predetermined header and sets an IP address of the ASNgateway 30 in the DA field.

The ASN gateway 30, when receiving this frame via the GRE tunnel,extracts the membership query report packet within the frame.Furthermore, the ASN gateway 30 specifies the sender BS2 on the basis ofthe IP address set in the SA field of the header of this frame, andrecognizes based on the MBS zone information table (FIG. 3) that the MBSzone, to which the BS2 belongs, is the MBS zone #1. Subsequently, theASN gateway 30 checks whether or not an address/zone pair of themulticast address (224.22.3.45) set in the group address field of theIGMP part in the frame and the MBS zone #1, is registered in themulticast group information table (FIG. 4) (S1303).

The ASN gateway 30, when confirming that the address/zone pair of themulticast address (224.22.3.45) and the MBS zone #1 is not registered,adds a new record to the multicast group information table and registersthese pieces of information (S1304). To be specific, such a new recordis added to the multicast group information table that [224.22.3.45] isset in the multicast address field, [MBS zone #1] is set in the MBS zonefield, and [BS2] is set in the last responder field.

The ASN gateway 30, thereafter, attaches the L2 header to the extractedmembership query report packet and thus transmits the packet to the edgerouter 20 (S1305). Hereat, the ASN gateway 30 sets an IP address of thegateway itself in the SA field of the IP header within this packet. Withthis address setting, the edge router 20 receiving this packetrecognizes from the IP address set in the SA field that the ASN gateway30 participates in the multicast group (224.22.3.45).

Next, a process, in which the plurality of mobile terminals MS3 and MS4in the same MBS zone transmits the JOIN message, will be explained.

At first, an assumption is that the MS3 desires to participate in amulticast group (224.0.10.15) and wirelessly transmits the JOIN messagein which to set the multicast address (224.0.10.15) (S1310).

The BS5, upon receiving this JOIN message signal, extracts themembership query report packet from this signal. The BS5 transmits aframe generated by attaching predetermined headers to the membershipquery report packet to the ASN gateway 30 via the GRE tunnel (S1311). Atthis time, the BS5 sets an IP address of the base station itself in theSA field of the predetermined header, and sets an IP address of the ASNgateway 30 in the DA field.

The ASN gateway 30, when receiving the frame via the GRE tunnel,extracts the membership query report packet within the frame. Further,the ASN gateway 30 specifies the sender BS5 on the basis of the IPaddress set in the SA field of the header of this frame, and recognizesfrom the MBS zone information table that the MBS zone, to which the BS5belongs, is the MBS zone #2. Subsequently, the ASN gateway 30 checkswhether or not an address/zone pair of the multicast address(224.0.10.15) set in the group address field of the IGMP part in theframe and the MBS zone #2, is registered in the multicast groupinformation table (S1312).

The ASN gateway 30, when confirming that the address/zone pair of themulticast address (224.0.10.15) and the MBS zone #2 is not registered,adds a new record to the multicast group information table and registersthese pieces of information (S1313). Specifically, such a new record isadded to the multicast group information table that [224.0.10.15] is setin the multicast address field, [MBS zone #2] is set in the MBS zonefield, and [BS5] is set in the last responder field.

The ASN gateway 30, thereafter, attaches the L2 header to the extractedmembership query report packet and thus transmits the packet to the edgerouter 20 (S1314).

Next, it is assumed that the MS4 desires to participate in the samemulticast group (224.0.10.15) as the MS3 does and wirelessly transmitsthe JOIN message in which the multicast address (224.0.10.15) is set(S1320).

The BS4, upon receiving the JOIN message signal, extracts the membershipquery report packet from this signal. The BS4 sends the frame generatedby attaching the predetermined headers to the membership query reportpacket to the ASN gateway 30 via the GRE tunnel (S1321).

The ASN gateway 30, when receiving this frame via the GRE tunnel,extracts the membership query report packet in the frame. Moreover, TheASN gateway 30, as in the case of the MS3, specifies the sender BS4 ofthe frame, and recognizes that the MBS zone, to which the BS4 belongs,is the MBS zone #2. Subsequently, the ASN gateway 30 checks whether ornot the address/zone pair of the multicast address (224.0.10.15) set inthe group address field of the IGMP part in the frame and the MBS zone#2, is registered in the multicast group information table (S1322).

The ASN gateway 30, when confirming that the address/zone pair of themulticast address (224.0.10.15) and the MBS zone #2 has already beenregistered, updates a value in the last responder field in the targetrecord of the multicast group information table into [BS4] (S1323), andterminates the process. With this scheme, the ASN gateway 30, in thecase of receiving the JOIN message with respect to thealready-registered multicast group in the same MBS zone, does not sendthe JOIN message to the edge router 20.

Next, an operation, in such a case that the ASN gateway 30 checksexistence of the mobile terminal 60 participating in the multicast groupset in the multicast group information table, will be described withreference to FIG. 14. FIG. 14 is a sequence diagram showing a processwhen sending the GS-query message, wherein the system architectureillustrated in FIG. 1 is exemplified.

The multicast group information table of the ASN gateway 30 shall be ina status shown in FIG. 4. The ASN gateway 30 checks the existence ofeach of the multicast groups entered in the multicast group informationtable on a predetermined cycle.

To be specific, the ASN gateway 30, for checking the existence of themulticast group (224.0.10.15), generates the GS-query packet by settingthe multicast address (224.0.10.15) in the group address field of theIGMP part and in the DA field of the IP header part. The ASN gateway 30recognizes that the MBS zone registered with respect to the multicastaddress is the MBS zone #2, and specifies the BS4 and BS5 as thosebelonging to the MBS zone #2. The ASN gateway 30 specifies the GREtunnels generated in regard to the multicast address (224.0.10.15)between the specified base stations BS4, BS5 and the ASN gateway 30.

The ASN gateway 30 copies the GS-query packet generated earlier, andattaches predetermined headers set for a recipient base station to eachGS-query packet. The ASN gateway 30 sends the thus-generated frames tothe BS4 and BS5 via the respective GRE tunnels (S1401). The timinginformation set in the GRE header part of each of the frames transmittedcontains the same information.

The BS4 and BS5 receiving the frames simultaneously transmit theGS-query packets in these frames through predetermined wireless channelsat the timing specified by the timing information set in the GRE headerparts (S1402).

The MS3 and MS4 receive the GS-query packet sent from any one of the BS4and BS5, corresponding to their locations. The MS3 and MS4, because ofthe same data being transmitted substantially simultaneously from theBS4 and BS5 during their movements, receive only the data transmittedfrom any one of the base stations. Herein, such an example is given thatboth of the MS3 and MS4 continuously participate in the multicast group(224.0.10.15), and the MS3 sends, ahead of MS4, the membership queryreport packet as a response to the GS-query.

The MS3, for indicating the continuous participation in the multicastgroup (224.0.10.15), wirelessly transmits a query response message inwhich to set the multicast address (224.0.10.15) (S1403).

The BS5, when receiving this query response message signal, extracts themembership query report packet from this signal. The BS5 sends a framegenerated by attaching predetermined headers to the membership queryreport packet to the ASN gateway 30 via the GRE tunnel (S1404). Hereat,the BS5 sets a self IP address in the SA field of the predeterminedheader and sets an IP address of the ASN gateway 30 in the DA field.

The ASN gateway 30, when receiving the frame via the GRE tunnel,extracts the membership query report packet in the frame. The ASNgateway 30 refers to the multicast address (224.0.10.15) set in thegroup address field of the IGMP part within the frame. Furthermore, theASN gateway 30 specifies the sender BS5 of this frame, and recognizesfrom the MBS zone information table that the MBS zone, to which the BS5belongs, is the MBS zone #2. With this scheme, the ASN gateway 30, as aresponse to the GS-query message sent earlier, checks that the mobileterminal 60 continuing the participation exists in the MBS zone #2 withrespect to the multicast address (224.0.10.15).

After checking, the ASN gateway 30 specifies the BS4 and BS5 as the basestation belonging to the MBS zone #2, and specifies the GRE tunnelsgenerated about the multicast address (224.0.10.15) between thespecified BS4, BS5 and the ASN gateway 30. The ASN gateway 30 copies theextracted membership query report packet as it is, and, after attachingpredetermined headers to each of the copies of the packet, transmits thethus-generated frames to the BS4 and BS5 via the specified GRE tunnels(S1405). The timing information set in the GRE header part of each ofthe frames transmitted contains the same information.

The BS4 and BS5 receiving the frames simultaneously transmit themembership query report packets in these frames through predeterminedwireless channels at the timing specified by the timing information setin the GRE header parts (S1406).

The MS3 and MS4 receive the membership query report packets sent fromany one of the BS4 and BS5, corresponding to their locations. The MS4thereby recognizes no necessity of making a response to the GS-querypacket received earlier, and does not wirelessly transmit the queryresponse message. This is because the ASN gateway 30 checks a status ofthe participation in each of the multicast groups on every MBS zonebasis, and the other mobile terminal MS3 existing in the same MBS zonehas already notified of participation continuing information.

Given next is a description of a case in which the ASN gateway 30checks, based on the multicast group information table, the existence ofthe multicast group (224.22.3.45) about the MBS zone #1. Herein, such acase is exemplified that the MS1 existing in the MBS zone #1 cancels theparticipation in the multicast group (224.22.3.45).

The ASN gateway 30 generates, for checking the existence of themulticast group (224.22.3.45), the GS-query packet in which themulticast address (224.22.3.45) is set in the group address field of theIGMP part and in the DA field of the IP header part. The ASN gateway 30recognizes that the MBS zone registered with respect to the multicastaddress is the MBS zone #1, and specifies the BS1, BS2 and BS3 as thebase station belonging to the MBS zone #1. The ASN gateway 30 specifiesthe GRE tunnels generated with respect to the multicast address(224.22.3.45) between the specified BS1, BS2, BS3 and the ASN gateway30.

The ASN gateway 30 copies the GS-query packet generated earlier, andattaches predetermined headers set for the destination base station toeach GS-query packet. The ASN gateway 30 sends the thus-generated framesto the BS1, BS2 and BS3 via the respective GRE tunnels (S1410). Thetiming information set in the GRE header part of each of the framestransmitted contains the same information.

The BS1, BS2 and BS3 receiving the frames simultaneously transmit theGS-query packets in these frames through predetermined wireless channelsat the timing specified by the timing information set in the GRE headerparts (S1411).

The MS1 receives the GS-query packet sent from any one of the BS1, BS2and BS3, corresponding to its location. The MS1, because of cancellingthe participation in the multicast group (224.22.3.45), does not makeany response to the GS-query message.

The ASN gateway 30, if the response to the GS-query message transmittedearlier is not sent for a predetermined timeout period, temporarilydetermines that any mobile terminal 60 continuing the participation inthe multicast group (224.22.3.45) does not exist in the MBS zone #1(S1412). The ASN gateway 30 retransmits the same GS-query message as themessage transmitted earlier to the MBS zone #1 (S1413).

The ASN gateway 30, if there no response to the retransmitted GS-querymessage, determines that none of the mobile terminal 60 continuing theparticipation in the multicast group (224.22.3.45) exists in the MBSzone #1, and deletes a target record from the multicast groupinformation table. Subsequently, the ASN gateway 30 checks whether ornot the records associated with other MBS zones in regard to themulticast group (224.22.3.45) are registered in the multicast groupinformation table. The ASN gateway 30, when confirming that none of therecords associated with other MBS zones are registered therein, sendsthe LEAVE message about the multicast group (224.22.3.45) to the edgerouter 20.

The edge router 20 receiving the LEAVE message recognizes that the ASNgateway 30 has left the multicast group (224.22.3.45). This has the samemeaning as recognizing that the mobile terminals 60 existing under theASN gateway 30 include none of the mobile terminals 60 continuing theparticipation in the multicast group.

Note that a processing sequence in the case of transmitting the LEAVEmessage from the mobile terminal 60 is that when the ASN gateway 30receives the LEAVE message, the GS-query message described above issent. Subsequent processes are the same as those of the processingsequence shown in FIG. 14, and hence the explanation is omitted.

Next, an operation in such a case that the LEAVE message is sent fromthe mobile terminal 60 will be described with reference to FIGS. 15 and16. FIG. 15 is a sequence diagram showing a first processing patternwhen receiving the LEAVE message, and FIG. 16 a sequence diagram showinga second processing pattern when receiving the LEAVE message, whereinthe system architecture illustrated in FIG. 1 is exemplified.

To begin with, the first processing pattern when receiving the LEAVEmessage will be explained with reference to FIG. 15. Hereat, anassumption is that the multicast group information table of the ASNgateway 30 is in a status illustrated in FIG. 4, and only the MS3, MS4make a request for participating in a multicast group (224.0.10.15) fromthe MBS zone #2.

The MS3 wirelessly transmits a LEAVE message signal in which to set themulticast address (224.0.10.15) in order to leave the multicast group(224.0.10.15) (S1501).

The BS5, when receiving the LEAVE message signal, extracts a LEAVEpacket from this signal. The BS5 transmits a frame generated byattaching predetermined headers to this LEAVE packet to the ASN gateway30 via the GRE tunnel (S1502). At this time, the BS5 sets an IP addressof the base station itself in the SA field in the predetermined headerand an IP address of the ASN gateway 30 in the DA field.

The ASN gateway 30, upon receiving the frame via the GRE tunnel,extracts the LEAVE packet within the frame. Furthermore, the ASN gateway30 specifies the sender BS5 on the basis of the IP address set in the SAfield of the header of this frame, and recognizes based on the MBS zoneinformation table (FIG. 3) that the MBS zone, to which the BS5 belongs,is the MBS zone #2. Subsequently, the ASN gateway 30 extracts, from themulticast group information table (FIG. 4), the base station informationregistered in the last responder field of a record registered with anaddress/zone pair of the multicast address (224.0.10.15) set in thegroup address field of the IGMP part within the frame and the MBS zone#2 (S1503). Herein, the information representing BS4 is extracted fromthe last responder field of the record.

The ASN gateway 30, when confirming that the sender BS5 of the LEAVEmessage is different from the extracted last responder (BS4), determinesthat the mobile terminal 60 continuing to participate in the multicastaddress (224.0.10.15) still exists in the MBS zone #2, and terminatesthe process (S1504).

Thereafter, the MS3, for leaving the multicast group (224.0.10.15),wirelessly transmits the LEAVE message in which the multicast address(224.0.10.15) is set (S1510).

The BS4, when receiving the LEAVE message signal, extracts the LEAVEpacket from this signal. The BS4 transmits a frame generated byattaching predetermined headers to the LEAVE packet to the ASN gateway30 via the GRE tunnel (S1511). Hereat, the BS4 sets the self IP addressin the SA field within the predetermined header and the IP address ofthe ASN gateway 30 in the DA field.

The ASN gateway 30, upon receiving the frame via the GRE tunnel, as inthe case of the MS3, specifies the sender BS4 of this frame, andrecognizes based on the MBS zone information table (FIG. 3) that the MBSzone to which the BS4 belongs is the MBS zone #2. Subsequently, the ASNgateway 30 extracts, from the multicast group information table (FIG.4), the base station information (BS4) registered in the last responderfield of a record registered with an address/zone pair of the multicastaddress (224.0.10.15) set in the group address field of the IGMP partwithin the frame and the MBS zone #2.

The ASN gateway 30, when confirming that the sender BS4 of the LEAVEmessage is coincident with the extracted last responder (BS4),determines that there is a possibility in which none of the mobileterminal 60 continuing to participate in the multicast address(224.0.10.15) might exist in the MBS zone #2 (S1512). The ASN gateway 30thereby transmits the GS-query packet via the GRE tunnels respectivelyto the BS4 and BS5 belonging to the MBS zone #2 (S1513).

The processing is done in the same way as in the case of checking theexistence of the mobile terminal 60 participating in the multicast groupshown in FIG. 14. At this time, none of the mobile terminal 60continuing the participation in the multicast group (224.0.10.15) existsin the MBS zone #2, and hence any query response message to the GS-querypacket is not sent.

Accordingly, the ASN gateway 30 finally determines that none of themobile terminal participating in the multicast group (224.0.10.15)exists in the MBS zone #2, and deletes a record containing a zone/grouppair of the MBS zone #2 and the multicast group (224.0.10.15) from themulticast group information table (S1519). Subsequently, the ASN gateway30 checks whether or not records related to other MBS zones with respectto the multicast group (224.0.10.15) are registered in the multicastgroup information table. The ASN gateway 30, when confirming that noother MBS zones are registered, sends the LEAVE message about themulticast group (224.0.10.15) to the edge router 20 (S1520).

Next, the second processing pattern when receiving the LEAVE messagewill be explained with reference to FIG. 16. At this time, an assumptionis that the multicast group information table of the ASN gateway 30 isin the status illustrated in FIG. 4, and only the MS3 and MS4 make arequest for participating in the multicast group (224.0.10.15) from theMBS zone #2.

The MS4, for leaving the multicast group (224.0.10.15), wirelesslytransmits the LEAVE message in which the multicast address (224.0.10.15)is set (S1610).

The BS4, when receiving the LEAVE message signal, extracts the LEAVEpacket from this signal. The BS4 transmits a frame generated byattaching predetermined headers to the LEAVE packet to the ASN gateway30 via the GRE tunnel (S1602). Hereat, the BS4 sets the self IP addressin the SA field within the predetermined header and the IP address ofthe ASN gateway 30 in the DA field.

The ASN gateway 30, upon receiving the frame via the GRE tunnel, in thesame way as above, specifies the sender BS4 of this frame, andrecognizes based on the MBS zone information table (FIG. 3) that the MBSzone to which the BS4 belongs is the MBS zone #2. Subsequently, the ASNgateway 30 extracts, from the multicast group information table (FIG.4), the base station information (BS4) registered in the last responderfield of a record registered with an address/zone pair of the multicastaddress (224.0.10.15) set in the group address field of the IGMP partwithin the frame and the MBS zone #2.

The ASN gateway 30, when confirming that the sender BS4 of the LEAVEmessage is coincident with the extracted last responder (BS4),determines that there is a possibility in which none of the mobileterminal 60 continuing to participate in the multicast address(224.0.10.15) might exist in the MBS zone #2 (S1603). The ASN gateway 30thereby transmits the GS-query packet via the GRE tunnels respectivelyto the BS4 and BS5 belonging to the MBS zone #2 (S1604).

The processing is executed in the same way as in the case of checkingthe existence of the mobile terminal 60 participating in the multicastgroup shown in FIG. 14. At this time, the mobile terminal (MS3)continuing the participation in the multicast group (224.0.10.15) stillexists in the MBS zone #2. The MS3, upon receiving this GS-query packet,transmits the query response message (S1606). The ASN gateway 30determines from this response that the mobile terminal 60 continuing theparticipation in the multicast group (224.0.10.15) still exists in theMBS zone #2. The ASN gateway 30 transmits, in the same way as by theprocess shown in FIG. 14, the query response message as it is to therespective BS4 and BS5 in the MBS zone #2 (S1608).

Finally, the ASN gateway 30 updates the last responder field in themulticast group information table with the information on the basestation (BS5) that relayed the query response message (S1610), andterminates the process (S1611).

Operations and Effects in the Present Embodiment

Herein, the operations and effects of the MBS system in the presentembodiment will be described.

In the MBS system according to the present embodiment, when the JOINmessage (the request for participating in the predetermined multicastgroup) sent from the mobile terminal 60 is transmitted to the ASNgateway 30 through the base station 50, the information about the MBSzone to which the base station relaying the JOIN message belongs andwith the information about the base station relaying the JOIN messagewith respect to the multicast group at which the JOIN message istargeted are stored.

Then, when the LEAVE message (the request for leaving the predeterminedmulticast group) sent from the mobile terminal 60 is transmitted to theASN gateway 30 through the base station 50, it is checked whether or notthe base station relaying the LEAVE message is registered as the lastresponder in the multicast group information table with respect to themulticast group at which the LEAVE message is targeted. If theregistered last responder is coincident with the base station relayingthe LEAVE message, the GS-query message (for checking the existence) issent to the MBS zone to which this via-base-station belongs. From thisoperation, it is confirmed that no other mobile terminals continuing theparticipation in the same multicast group exist, and the target recordis deleted from the multicast group information table.

Whereas if the registered last responder is not coincident with the basestation relaying the LEAVE message, the process is terminated. This isbecause it can be recognized that other mobile terminals exist in theMBS zone to which that base station belongs.

With this scheme, according to the present embodiment, even in the caseof receiving the LEAVE message, if there is a high possibility thatother mobile terminals continuing their participation might exist in thesame MBS zone, the GS-query message for checking the existence may notbe transmitted, and hence the unnecessary traffic can be reduced.

This scheme, according to the present embodiment, enables the multicastcommunications to be efficiently performed in the MBS system targeted atthe mobile terminals 60 on the premise that these terminals are to move.

Modified Example

In the ASN gateway 30 described above, when receiving the JOIN message,the last responder field in the multicast group information table isupdated with the information about the base station relaying the JOINmessage, however, the information about the base station relaying theJOIN message may be entered in the last responder field only whenreceiving the JOIN message for the first time with respect to the sameMBS zone.

Even in the case of the operation being thus done, the existence of themobile terminal participating in the multicast group can be invariablychecked on every MBS zone basis by periodically checking the mobileterminal continuing the participation by use of the GS-query message.

Others

The disclosures of Japanese patent application No. JP2006-301594, filedon Nov. 7, 2006 including the specification, drawings and abstract areincorporated herein by reference.

1. A relay device transmitting a user data packet, containing a contentto be distributed to a mobile terminal, in which to set informationabout a multicast group associated with the content, to a plurality ofbase stations managed in the way of being grouped into zones, the relaydevice comprising: a storing unit storing information about a multicastgroup in which the mobile terminal participates, information about thebase station performing wireless communications with the mobile terminaland information about the zone to which the base station belongs in away that associates these items of information with each other; aextracting unit extracting, when receiving a LEAVE packet of a requestfor leaving the multicast group from the mobile terminal via any one ofthe plurality pf base stations, the information about the base station,which is stored in the way of being associated with the leave targetmulticast group and with the zone to which the via-base-station belongs,from the storing unit; a determining unit determining whether or not itis required to transmit a query packet for checking existence of themobile terminal continuing the participation in the leave targetmulticast group by comparing the base station extracted by theextracting unit with the base station that relays the received LEAVEpacket; and a transmitting unit transmitting the user data packetcontaining the query packet to each of the base stations belonging tothe zone whose information is stored together with the information aboutthe leave target multicast group in the storing unit on the basis of aresult of the determination made by the determining unit.
 2. A relaydevice according to claim 1, further comprising an updating unitupdating, when receiving a JOIN packet of a request for participating inthe multicast group from the mobile terminal via any one of theplurality of base stations, the information about the base station thatis stored in the storing unit in the way of being associated with thezone to which the via-base-station belongs and with the participationtarget multicast group, with the information about the base station thatrelays the JOIN packet.
 3. A relay device according to claim 1, furthercomprising a deleting unit deleting, if a response packet is notreceived when the transmitting unit has transmitted the user data packetcontaining the query packet, a record containing the leave targetmulticast group, the base station that relays the LEAVE packet and thezone to which the base station belongs in the way of their beingassociated with each other from the storing unit.
 4. A relay deviceaccording to claim 3, further comprising a transferring unit checking,when the record is deleted by the deleting unit, existence of otherrecords stored in the storing unit in the way of being associated withthe multicast group contained in the deleted record, transferring thereceived LEAVE packet to a high-order device if no other records exist,and transferring none of the received LEAVE packet if other recordsexist.
 5. A wireless communication system comprising: a plurality ofbase stations managed in the way being grouped into zones; and a relaydevice transmitting a user data packet, containing a content to bedistributed to a mobile terminal, in which to set information about amulticast group associated with the content, to any of the plurality ofbase stations, the relay device comprising: a storing unit storinginformation about a multicast group in which the mobile terminalparticipates, information about the base station performing wirelesscommunications with the mobile terminal and information about the zoneto which the base station belongs in a way that associates these itemsof information with each other; a extracting unit extracting, whenreceiving a LEAVE packet of a request for leaving the multicast groupfrom the mobile terminal via any one of the plurality pf base stations,the information about the base station, which is stored in the way ofbeing associated with the leave target multicast group and with the zoneto which the via-base-station belongs, from the storing unit; adetermining unit determining whether or not it is required to transmit aquery packet for checking existence of the mobile terminal continuingthe participation in the leave target multicast group by comparing thebase station extracted by the extracting unit with the base station thatrelays the received LEAVE packet; and a transmitting unit transmittingthe user data packet containing the query packet to each of the basestations belonging to the zone whose information is stored together withthe information about the leave target multicast group in the storingunit on the basis of a result of the determination made by thedetermining unit, wherein in the plurality of base stations, each of thebase stations belonging to the same zone receives the user data packettransmitted from the transmitting unit of the relay device, andwirelessly transmits the query packet contained in the user data packetsubstantially simultaneously with the other base stations belonging tothe same zone.
 6. A wireless communication system according to claim 5,wherein the relay device further comprises an updating unit updating,when receiving a JOIN packet of a request for participating in themulticast group from the mobile terminal via any one of the plurality ofbase stations, the information about the base station that is stored inthe storing unit in the way of being associated with the zone to whichthe via-base-station belongs and with the participation target multicastgroup, with the information about the base station that relays the JOINpacket.
 7. A wireless communication system according to claim 5, whereinthe relay device further comprises a deleting unit deleting, if aresponse packet is not received when the transmitting unit hastransmitted the user data packet containing the query packet, a recordcontaining the leave target multicast group, the base station thatrelays the LEAVE packet and the zone to which the base station belongsin the way of their being associated with each other from the storingunit.
 8. A wireless communication system according to claim 7, whereinthe relay device further comprises a transferring unit checking, whenthe record is deleted by the deleting unit, existence of other recordsstored in the storing unit in the way of being associated with themulticast group contained in the deleted record, transferring thereceived LEAVE packet to a high-order device if no other records exist,and transferring none of the received LEAVE packet if other recordsexist.
 9. A multicast relay method of transmitting a user data packet,containing a content to be distributed to a mobile terminal, in which toset information about a multicast group associated with the content, toa plurality of base stations managed in the way of being grouped intozones, the multicast relay method comprising: a storing step of storing,in a storing unit, information about a multicast group in which themobile terminal participates, information about the base stationperforming wireless communications with the mobile terminal andinformation about the zone to which the base station belongs in a waythat associates these items of information with each other; anextracting step of extracting, when receiving a LEAVE packet of arequest for leaving the multicast group from the mobile terminal via anyone of the plurality pf base stations, the information about the basestation, which is stored in the way of being associated with the leavetarget multicast group and with the zone to which the via-base-stationbelongs, from the storing unit; a determining step of determiningwhether or not it is required to transmit a query packet for checkingexistence of the mobile terminal continuing the participation in theleave target multicast group by comparing the base station extracted inthe extracting step with the base station that relays the received LEAVEpacket; and a transmitting step of transmitting the user data packetcontaining the query packet to each of the base stations belonging tothe zone whose information is stored together with the information aboutthe leave target multicast group in the storing unit on the basis of aresult of the determination made in the determining step.